1. Field of the Invention
The present invention relates to a motor control device that controls a motor, a motor control method, and an image forming apparatus.
2. Description of the Related Art
Image forming apparatuses such as a printer have been known that form images onto the sheets conveyed by a sheet conveying unit with an image forming unit using the inkjet printing structure or the electrophotography.
Stepping motors have been used as a driving unit for driving the sheet conveying unit. Technologies using a direct current (DC) motor are also widely known in place of technologies using the stepping motors. When using a DC motor, the rotational speed of the DC motor is detected and feedback control is performed so that the detected rotational speed becomes the target speed. In this manner, the sheet conveying unit is driven so that a sheet is conveyed at a predetermined speed.
A typical DC motor has a smaller average power consumption and a larger maximum current than a stepping motor. If an excessive load is applied to the DC motor and the motor enters a lock state (a state in which the DC motor becomes unable to rotate and stops), overcurrent flows through a switching device such as a field-effect transistor (FET) in a control circuit of the DC motor. This may cause damage to the switching device.
In Japanese Patent Application Laid-open No. 2002-347296, a technology is disclosed in which determination is performed whether the DC motor is in the lock state based on a pulse signal for driving the DC motor. Specifically, in the technology disclosed in Japanese Patent Application Laid-open No. 2002-347296, every time the duty of the pulse signal is controlled at constant intervals, determination is performed whether the duty reaches the maximum value. The number of times the duty reaches the maximum value is counted by a counter. Unless the count value of the counter reaches a predetermined value, the DC motor is not stopped although the duty reaches the maximum value and increases the count value of the counter. If the count value of the counter reaches the predetermined value, it is determined that the DC motor is in the lock state, so the voltage application to the DC motor is stopped.
Typical image forming apparatuses include two drive states of the DC motor in the sheet conveying unit: a normal drive state and a position holding state. In the normal drive state, the sheet conveyance speed is controlled to an acceleration speed, a deceleration speed, or a constant speed. In the position holding state, control is performed to hold the rotational position of the DC motor for slacking the sheet during conveyance. That is to say, in the position holding state, the DC motor is driven so that the position of the sheet is held.
There is a problem, however, in that if the control is performed without determining whether the drive state of the DC motor is the normal drive state or the position holding state, the drive state of the DC motor controlled in the position holding state may be detected erroneously as being in the lock state. This stops voltage application to the DC motor although the motor is in the position holding state, whereby the rotational position of the DC motor, that is, the position of the sheet cannot be held.
In view of the above, there is a need to avoid an erroneous detection of a lock state of a motor actually in a position holding state.